Life in the well-manicured London locale of Hexam Place is not as placid and orderly as it appears. Behind the tranquil gardens and polished entryways, relationships between servants and their employers are set to combust.    Henry, the handsome valet to Lord Studley, is sleeping with both the Lord's wife and his university-age daughter. Montserrate, the Still family's lazy au pair, is helping to hide Mrs. Still's illicit affair with a television actor--for a small fee. June, the haughty housekeeper to a princess of dubious origin, is hard at work forming a "society" for servants to address complaints about their employers. Meanwhile, a disturbed gardener, Dex, believes a voice in his cellphone is giving him godlike instructions--that could endanger the lives of all who reside in Hexam Place.    A deeply observed and suspenseful update to the upstairs/downstairs genre, The Saint Zita Society is Ruth Rendell at her incisive best.